Sebastian Barry's fifth novel begins with a question: "What is the sound of an eighty-nine-year-old heart breaking?" The heart in question belongs to a woman named Lilly Bere, who fled to America from Dublin at the end of the first world war, in the hope of escaping her fate, and found another, just as bloody, waiting there for her.

Lilly is another part of a fictional family that Barry has imagined in earlier writing, the daughter of Thomas Dunne, chief superintendent of the Dublin military police, whose story he told in his celebrated play The Steward of Christendom. Her association with the British crown, her childhood in Dublin castle, is dealt with only briefly, but it colours all that follows. Lilly is stained with Irish politics from birth, just as surely as the nonagenarian narrator of Barry's last Booker-shortlisted book, The Secret Scripture. Roseanne McNulty was incarcerated in Roscommon mental asylum, telling her intimacies to her psychiatrist. Unlike Roseanne, Lilly has got out of civil war Ireland, but she has carried much of the madness with her across the Atlantic.

Her broken heart makes only a "small, slight sound", she suggests, at her advanced age, not least because it has been broken several times already. The book is narrated in the quiet wildness of grief over the course of 17 days, each one a chapter, as Lilly mourns her grandson. His suicide after his return from the Gulf war is only the most meaningless of all the casualties of history whom Lilly has loved and lost in what amounts to "Oldest Surviving Living Unionist Widow Tells All".

Lilly fled first with her awkward young lover, Tadg Bere, friend to her brother Willie Dunne, whose death in the trenches at Picardy Barry detailed in his 2005 novel A Long Long Way. Tadg had enlisted for the Black and Tans and become a wanted man to the emergent IRA. America looks to the lovers in their desperation like the promised land. On Canaan's side, however, they find no escape from homegrown enmities. Tadg is murdered, Lilly spared to other hope and other grief.

Barry captures this unfolding life, punctuated by the wider events of history  the second world war, the civil rights movement, Vietnam  with a vivid lyricism. He is fascinated by the weights and measures of memory, the way a single afternoon on a rollercoaster or sunlight momentarily falling on water can occupy as much space, looking back, as an assassination or the birth of a child. "We may be immune to typhoid, tetanus, chickenpox, diphtheria, but never memory. There is no inoculation against that," Lilly notes.

At certain points, in wanting Lilly's life to be symptomatic of her times  she finds work as housekeeper to a political family and cooks pecan pie for Martin Luther King  the historical narrative threatens to overwhelm the intimacies of her life. By the time Barry gets to her grandson  "He liked Bob Dylan, and sang his songs quite tunelessly around the house. The atom bomb haunted his dreams"  he seems to be risking a kind of historical shorthand. There is a neatness, too, to the plotting as the story eventually unfolds that provides a shape and closure that the messiness of such history rarely affords.

But the little epiphanies and stirrings of love that Lilly Bere clings to as things falls apart are what keep both her and her story honest. Barry has a poet's sense of quickening pulse, and his writing comes alive in details. "And there we were as private as foxes, strolling under the low branches," he writes of Lilly's recollection of her first passionate encounter with her second husband. These moments properly magnify loss, which is the book's persistent theme, and from which there often seems no respite. It is men, in the novel's world, who do most of the dying, in ways that they can seldom fathom; the women are left to tell the tale.

In his fifth novel, Sebastian Barry takes up the story of another of the Dunnes, the family whose members have appeared in Annie Dunne and A Long, Long Way and in his play The Steward of Christendom. Eighty-nine-year-old Lilly Bere recounts the events of her life as though mesmerised by the vivid incompleteness of a remembered dream; in this, although her grasp is surer and her memories less fantastical, she resembles the narrator-protagonist of Barry's award-winning previous novel, The Secret Scripture. Both Lilly and Roseanne Clear before her are vastly aged, determinedly contemplative and driven to make what sense they can of the violence that has befallen them, sundering them from family and loved ones and the lives they had hoped to have. But while Roseanne's testimony was fashioned within the confines of a mental institution, Lilly's final reckoning takes place from the apparent safety and tranquillity of the American coastline, the "promised land" suggested by the novel's title.

It is, we rapidly discover, only a comparative sort of safety, and a tranquillity that can be shattered in a moment. Lilly writes in the aftermath of the suicide of her grandson, whose life unravelled irreparably after his service in the Gulf war made him lose faith in the terms "victory" and "defeat". Consumed with grief, carrying "in my skull a sort of molten sphere instead of a brain", she is compelled to make her account despite a hatred of "pens and paper and all that fussiness" and a mistrust of "the heavy-hearted tales of history". That her life has been shaped to a painfully minute degree by those heavy-hearted tales becomes evident; but so too does her natural unwillingness to submit to their flattening narratives, so much so that when she has finished her account, she is resolved to bring an end to her days by "some quiet method".

Quietness has been her preferred mode, a choice foisted upon her by a fugitive life. The daughter of a high-ranking policeman in the Dublin Metropolitan Police (her father's story forms the basis of The Steward of Christendom), she already embodies a deeply ambivalent and problematic strand of Irish history, animating the divisions that rapidly worsened between the first world war and the formation, in 1922, of the Irish Free State. When, as a teenager, she meets Tadg Bere, returned from the war and now an auxiliary police officer in the reviled Black and Tans, they quickly fall foul of the politicised lawlessness spreading through Ireland; and when, following an ambush in which a group of rebels are killed, their names appear on a death-list, they flee Dublin by night and head for America. But their assumed names and furtive progress from New York to the "glittering Canaan" of Chicago is of little avail: only months after their arrival, Tadg is gunned down in an art gallery and Lilly, covered in his blood and stunned by "the colossal ungenerosity" of his murder, is forced back into escape and anonymity.

Barry resists filling in complex historical detail with a heavy hand, although he is more heavy-hearted than might meet with Lilly's approval. His method is to imply a dreadful strangeness rather than a straightforward working-out; conflict and tumult blind-side his characters rather than staring them full in the face. "Tanks. Wounds received. Nothing," reports Lilly's friend Mike Scopello tersely, when asked about the Purple Heart he received in the second world war; elsewhere, America's brimming racial tensions and its participation in the Vietnam war are similarly obliquely sketched, their force and magnitude evident from the damage and alienation that they leave in their wake. Sometimes those effects are bizarre and mysterious: in the sinuous strand of the novel that charts Lilly's marriage to policeman Joe Kinderman, his subsequent abandonment of her is eventually revealed to be the result of a painful neurosis that is all the more powerful for its unpredictability and oddness.

By anybody's reckoning, Lilly's life is a traumatic one, encompassing multiple bereavements and separations, material hardship, numerous upheavals and unrelieved exile from an oppressed and divided homeland. Her indomitability  she is, she tells us, "thankful for my life, infinitely"  derives in part from the very invisibility and stoicism that she has had to cultivate and for the joy in small reliefs and pleasures to which that has led. Paradoxically, shrinking her life to escape the assassin's gaze has induced in Lilly a deep appreciation of America's vastness and mobility, a mental relief from claustrophobia. Arriving in New York, slinking through the streets with Tadg, she is at once terrified and awed: "I almost laughed at the memory of Dublin, with its low houses, their roofs tipped like deferential hats to the imperious rain." Much later, in Cleveland, in a marvellously conjured episode in which Lilly visits Luna Park with Joe, "the generous American sky threw its arms open above us, and above the brightened factories, and the stretching wilderness of the human streets."

Barry's prose is overwhelmingly poetic, its lyricism yielding a seemingly endless series of potent and moving images: Lilly's shoreline home, engulfed by "those long-limbed creaturely fogs that walk in against the Hamptons like armies"; her brother Willie Dunne home on leave from France, "disguised by the thin dust of terror he carried on him"; Lilly's mind careering through her past like "an unbroken pony". This concentration on isolating tiny fragments of experience and apprehension makes for an intense and immersive read, one in which brutal events are cast in a diffuse light that gives them an almost mythic quality. But the narrative's dreamlike qualities do not eclipse Barry's determination to scrutinise the less travelled byways of history and to give a voice to their buffeted, battered but nonetheless enduring victims.